Wireless Mesh Networks - DISI, University of...

Wireless Mesh Networks

Renato Lo Cignowww.disi.unitn.it/locigno/teaching

Part of this material (including some pictures) features and are freely reproduced from:

“Ian F.Akyildiz, Xudong Wang,Weilin Wang, ‘Wireless mesh networks: a survey’, Computer Networks 47 (2005), Elsevier“

Thanks also to Gianni Costanzi for checks and providing figures

[email protected] Nomadic Communications: Wireless Mesh Networks 2

Ad-Hoc and WMN

• Ad-Hoc network – non permanent

– general purpose or specific (sensors)

– single or multi-hop, normally mobile

– may require routing (see AODV and OLSR)

• Wireless Mesh Networks (WMN)– more structured than Ad-Hoc

– may be hierarchical

– semi-permanent, some nodes are fixed

– requires routing


WMN: a general view


A Mesh – Ad-hoc network

• Ad-Hoc can be meshed– non single broadcast channel

– multi-hop require routing


Hierarchical meshes


Hierarchical meshes

• Capacity of the backbone

• Routing strategies – Gateway selection

• client level

• backbone level

• Backbone of fixed nodes– multi-km links -> easy and cheap coverage

– replace wireless “closed” backbones

– Nomadic access vs. static access


Domestic Mesh

• Simplify home cabling

• Can support anti-intrusion

• Distribute e.g. IPTV


Building automation

• Simplify cabling

• Allow central control – vs. pure sensor/actuator

networking where information is not propagated

• Simple, static routing (but does not work!)

• Reliability concerns


Multi-home meshes

• Community networks

• Social networks

• SOHO support

• Nomadic access


Vehicular-metropolitan networks


Vehicular-metropolitan networks

• Mainly infrastructure-to-vehicle – cooperative driving is a different (though related)

story

• Traffic control & congestion management– A22 is “selling” as the “future” 73 messaging panels on

close to 300 km ...

• Turism, advertisement, local information• Nomadic communication with pedestrians too

• In U.S. some commercial experiments are already available


Train & Planes networks

• Cellular networks? – capacity problems in “dense” environments

– cannot “reach” planes

– problems with very high speed

• Collect the traffic locally then interconnect from a single – non energy constrained point


Mesh project & sites

• Community Networks & around– Seattle Wireless (http://www.seattlewireless.net/)– Roofnet at MIT (http://pdos.csail.mit.edu/roofnet/) – TFA at Rice (http://tfa.rice.edu) – Tuscolo Mesh (http://tuscolomesh.ninux.org/joomla)– Georgia Tech

(http://www.ece.gatech.edu/research/labs/bwn/mesh/index.html)

– ...– Pergine Valsugana – ... – Trentino Networks


Mesh: Basic scenarios (1)

• Extended WLAN access• Simple configuration

– no routing

• Simple 802.11 handover support

• Double radio guarantees good performance

• Single radio creates resource conflicts – 3 BSS on the same channel– suitable for low-cost low-performance



• Extended WLAN access

• Routing required• Simple 802.11

handover support• Double radio

guarantees good performance

• Single radio creates serious resource conflicts – n+1 BSS on the same channel

• WDS is broadcast

• A(GW) can be a bottle-neck




Mesh: Basic scenarios (3)• Extended WLAN access• Basic infrastructuring• Single radio operation very

difficult

• Multiple external gateways– sophisticated, flow-based routing

• Non standard handover support – flow based routing requires exporting the context– address management require coordination

• WDS may be multi-hop– How many channels?

• Point-to-point and broadcast channels in WDS



Moving between BSS

belonging to different Mesh/WDS

• Address management (DHCP) is a problem

• Flow-based routing may be impossible

• Joining/splitting of partitions is an open issue


Mesh – Ad-Hoc: AODV

Ad-hoc On-demand Distance Vector routing –rfc3561

• DV (see RIP) protocol for next-hop based routing

• On-Demand: maintains routes only for nodes that are communicating

• Must build routes when requested• Route Request (RREQ) are flooded through the

network• Nodes set-up reverse path pointers to the

source– AODV assumes symmetric links



• The intended receiver sends back a Route Reply (RR)

• RR follow the reverse path set-up by intermediate nodes (unicast) establishing a shortest path route memorized by intermediate nodes

• Paths expire if not used – protocol & transmission overhead– guarantee of stability in dynamic, non reliable

networks

• Usual DV problems– count to infinity, slow convergence, ...



• Next-hop based (other proposals are based on source routing)

• “Flat” protocol: all nodes are equal

• Can manage only one route per s-d pair – can be inefficient in presence of highly variable link

quality and persistence

• Good for sporadic communications

• Bad for high mobility– slow convergence

– difficulty in understanding topology changes.


Mesh – Ad-Hoc: AOMDV

Ad-Hoc On-demand Multipath Distance Vector Routing in Ad Hoc Networks– An extension to AODV

– AOMDV computes multiple loop-free and link-disjoint paths

– Using “Advertised Hop-count” guarantees Loop-freedom• A variable, which is defined as the maximum hop count for all the

paths. A node only accepts an alternate path to the destination if it has a lower hop count than the advertised hop count for that destination

– Link-disjointness of multiple paths is achieved by using a particular property of flooding

– Performance comparison of AOMDV with AODV shows that • AOMDV improves the end-to-end delay, often more than a factor of

two

• AOMDV reduces routing overheads by about 20%


Mesh – Ad-Hoc: OLSR

Optimized Link-State Routing Protocol (rfc3626)

• Proactive, link-state routing protocol

• Based on the notion of MultiPoint Relay (MPR)

• Three main components:– Neighbor Sensing mechanism

– MPR Flooding mechanism

– topology Discovery (diffusion) mechanism.

• Auxilary features of OLSR:– network association - connecting OLSR to other networks



Basic neighbor sensing:

• periodic exchange of HELLO messages;

• HELLO messages list neighbors + "neighbor quality“– HEARD - link may be asymmetric

– SYM - link is confirmed to be symmetric

– MPR - link is confirmed to be symmetric AND neighbor selected as MPR

• Providing:– topology information up to two hops

– MPR selector information notification



• Each node selects from among its neighbors an MPR set such that– an emitted flooding message, relayed by the MPR

nodes, can be received by all nodes in the 2-hop neighborhood

• Goals:– reduce flooding overhead (select minimal sets)

– provide optimal flooding distances



• Exchanges topology information with other nodes of the network regularly

• MPRs announce their status periodically in control messages.

• In route calculation, the MPRs are used to form the route from a given node to any destination in the network

• Uses MPRs to facilitate efficient flooding of control messages


Mesh Networks: 802.11s

• Working group to deliver a standard for 802.11(& around) base Mesh Networks– Interactions with 802.11p dedicated to vehicular

networks

• Tries to define a framework to support a Mesh network as a standard extended WLAN with routing that goes beyond the standard minimum spanning tree of 802.1 interconnection


Device Classes in 802.11s

• Mesh Point (MP)– a point able to relay messages

• Mesh AP (MAP)– a MP able to provide services to STAs

• Mesh Portal (MPP)– a MAP connected to a wired LAN

– normally called a gateway and assumed to access the internet


Routing in 802.11s

• Hybrid Wireless Mesh Protocol (HWMP) -Mandatory– AODV derived link-state protocol

– Based on trees for proaction and efficiency

– Add on-demand features (like AODV)

• Radio Aware OLSR (RA-OLSR) – Optional– Radio aware metrics added to MPRs in OLSR

– optional fish-eye routing capabilities

– association and discovery protocols for topology discovery and buildup

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